The studies comprising this proposal are directed at establishing the nature and structural organization of the nuclear RNA precursors to immunoglobulin light chain mRNA at determining the molecular events in the post-transcriptional processing of these nuclear RNA in the biogenesis of immunoglobulin light chain mRNA. The requisite first step in such studies is the development of means for the specific isolation of nuclear RNA containing immunoglobulin light chain mRNA sequences. We have developed a general method for cloning complementary DNA (cDNA) generated from poly(A)-containing mRNA. We have constructed clones from purified MOPC 21 immunoglobulin kappa light chain mRNA for probes in these studies on the nuclear RNA precursors to kappa light chain mRNA. We have now defined the following pathway for the biogenesis of kappa light chain mRNA: 40S yields 24S yields 13S transport to the cytoplasm yields 13S kappa mRNA. Proposed studies are directed at characterizing the events and RNA species in this pathway. These studies represent an innovative approach for the analysis of gene expression in eukaryotic cells using the powerful resolution afforded by recombinant DNA cloning. It is anticipated that these studies should not only provide substantial insights into the nature and expression of immunoglobulin genes, but into the general nature of gene structure and expression in eukaryotic organisms as well.